The current mainstream of semiconductor devices resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. While modern automobiles are equipped with electronic control systems, semiconductor devices are employed around automobile engines where they are frequently exposed to elevated temperatures. It is required to insure device operation at elevated temperatures.
In semiconductor encapsulating epoxy resin compositions, halogenated epoxy resins combined with antimony trioxide (Sb2O3) are often included in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect.
However, when a semiconductor device encapsulated with an encapsulant comprising a halogenated epoxy resin and antimony trioxide is exposed to elevated temperatures for a long period of time, halide ions promote formation of metal compounds in the joint areas between gold wires and aluminum pads on the silicon chip, detracting from mechanical strength and electrical properties.
Under these circumstances, studies have been conducted on the use of hydroxides such as Al(OH)3 and Mg(OH)2 or phosphorus-based fire retardants such as red phosphorus and phosphates in place of halogenated epoxy resins and antimony trioxide. Unfortunately, various problems arise from the use of these alternative compounds. The hydroxides such as Al(OH)3 and Mg(OH)2 have less flame retardant effects and must be added in larger amounts in order that epoxy resin compositions be flame retardant. Then the viscosity of these compositions increases to a deleterious level to molding, causing molding defects such as voids and wire flow. On the other hand, the phosphorus-based fire retardants such as red phosphorus and phosphates added to epoxy resin compositions can be hydrolyzed to generate phosphoric acid when the semiconductor devices are exposed to hot humid conditions. The phosphoric acid generated causes aluminum conductors to be corroded, detracting from reliability.
Compositions comprising an epoxy resin and a curing agent cure into products which can be thermally degraded to generate organic acids such as acetic acid and formic acid, giving negative impact on the reliability of semiconductor devices. Then, resin compositions having a low epoxy equivalent, a high aromatic ring content and good thermal stability as well as high flame retardance are used. However, their cured products have a low glass transition temperature (Tg), and thus exhibit less satisfactory electrical properties at temperatures higher than the Tg because ionic impurities become more active or mobile.
It would be desirable if semiconductor-encapsulating epoxy resin compositions cure into products which have improved heat-resistant reliability and moisture-proof reliability in that ionic impurities become less during high-temperature, long-term storage. It was proposed to add hydrotalcite compounds to the compositions for the purposes of trapping or scavenging ionic impurities and neutralizing the acidity of cured compositions. See Japanese Patent Nos. 2,501,820, 2,519,277, 2,712,898, and 3,167,853, JP-B 6-051826, JP-A 9-118810, JP-A 10-158360, JP-A 11-240937, JP-A 11-310766, JP-A 2000-159520, JP-A 2000-230110, and JP-A 2002-080566. The hydrotalcite compounds, however, fail to overcome the problem of epoxy resin compositions having added thereto phosphorus-based flame retardants such as phosphazene compounds that the phosphorus-based flame retardants are decomposed to generate phosphoric acid when semiconductor devices are exposed to high temperatures. That is, there is a certain limit for the hydrotalcite compounds to completely convert the phosphoric acid into a non-detrimental form.
Japanese Patent No. 2,843,244 proposes an epoxy resin composition using a flame retardant having red phosphorus coated with a surface layer of SixOy although it still lacks moisture-proof reliability. JP-A 10-259292 discloses an epoxy resin composition in which a cyclic phosphazene compound is used in an amount to give 0.2-3.0% by weight of phosphorus atoms based on the total weight of other components excluding the filler. To provide flame retardance, a substantial amount of the cyclic phosphazene compound must be added to the epoxy resin composition, which can cause ineffective cure and a lowering of electrical resistance in a high-temperature environment. Although the cyclic phosphazene compound is relatively stable to heat, long-term exposure to high temperatures can cause the phosphazene compound to be decomposed to generate phosphoric acid, giving negative impact to the high-temperature operation of semiconductor devices.
Further, JP-A 2003-138102 discloses a semiconductor encapsulating, flame retardant epoxy resin composition comprising as essential components, (A) an epoxy resin, (B) a curing agent, (C) an inorganic filler, (D) a molybdenum ingredient having zinc molybdate supported on an inorganic filler, and (E) a phosphazene compound having the average compositional formula (i):
wherein X is a single bond or a group selected from among CH2, C(CH3)2, SO2, S, O, and O(CO)O, Y is OH, SH or NH2, R1 is a group selected from among C1-C4 alkyl and alkoxy groups, NH2, NR2R3 and SR4, wherein each of R2, R3 and R4 is a hydrogen atom or an C1-C4 alkyl group, subscripts d, e, f and n are numbers satisfying 0≦d≦0.25n, 0≦e<2n, 0≦f≦2n, 2d+e+f=2n, and 3≦n≦1000. This epoxy resin composition is free of bromides such as brominated epoxy resins and antimony compounds such as antimony trioxide, is effectively moldable, and cures into a product having improved flame retardance and moisture-proof reliability. However, it does not overcome the problem of epoxy resin compositions having phosphorus-based flame retardants such as phosphazene compounds added thereto that the phosphorus-based flame retardants can be decomposed to generate phosphoric acid compounds having negative impact on the reliability of semiconductor devices, when semiconductor devices are exposed to high temperatures.